What I think would be cool is to take elements from "Where No Man Has Gone Before" and make Gary Mitchell the villain for the next movie. Instead of getting mental abilities as he did in the TV episode, perhaps he contracts a virus that makes him think he has mental abilities. He goes rogue and takes the colony ship he escorting hostage and threatens attacks on Klingon outposts. Kirk and the Enterprise have to hunt him down. Kirk would have to face the difficult decision about killing his best friend to save the colonists and prevent war with the Klingons.